1. Field of the Invention.
The present invention relates to improved methods of drying pipelines containing water which are to transport hydrate forming gaseous fluids.
2. Description of the Prior Art.
After a pipeline for the transportation of light hydrocarbons such as natural gas has been repaired or constructed and hydro-tested or otherwise exposed to water, it is mandatory that water remaining in the pipeline be removed. The reason for this is that light hydrocarbon gases form hydrates with water which can and often do reduce or block the flow of gases through pipelines.
Gas hydrates are similar to ice crystals and form by the combination of light hydrocarbon gases and water under certain temperature and pressure conditions. The pipelines have heretofore been dried by passing a water absorbing gas such as dry air or nitrogen through the pipeline, by pulling a vacuum on the pipeline, by passing methanol through the pipeline or by a combination of the foregoing techniques. A successful procedure for drying a pipeline which has been used heretofore is to pass methanol through the pipeline.
The methanol is usually separated in stages by pipeline pigs which allows the water in the pipeline to be exposed to successive methanol contact which brings about successive dilutions of the methanol-water mixture remaining in the pipeline. Such methanol treatments have resulted in films of methanol-water mixtures remaining in pipelines containing very small amounts of water. However, even very small amounts of water may be sufficient to cause hydrate formation in very long, cold pipelines. For example, a 24 inch pipeline which is about 700 miles long will be left with a film of methanol and water mixture on its inside surfaces after a methanol pigging treatment of the type described above. Assuming the film to be about 0.1 millimeter thick, the total volume of the methanol-water mixture left in the pipeline is about 7,700 cubic feet of liquid, most of which is methanol. However, a property of methanol-water mixtures is that the methanol evaporates faster than the water. Thus, as evaporation of the methanol-water mixture left in the pipeline takes place over time, the mixture constantly increases in water concentration. Accordingly, there is a possibility that the methanol will be stripped from the mixture remaining in the pipeline leaving the mixture containing a high concentration of water which can cause the formation of hydrates in the pipeline.
Thus, there is a need for improved methods of drying pipelines containing water which are to transport hydrate forming gaseous fluid.